The liquid crystal display device is a display device that is used for a notebook computer, a personal computer, a smart phone, a TV and the like, and its characteristics are also improved every year according to an expansion of demand for the liquid crystal display device.
A liquid crystal panel of a liquid crystal display device as a non-light-emitting element structurally requires a backlight unit. The backlight unit is made up of variety of optical systems. Further, the backlight unit uses an optical film of periodic arrangement in order to improve brightness.
FIG. 1 is a diagram schematically illustrating a configuration of a conventionally developed liquid crystal display device.
As illustrated in FIG. 1, the backlight unit 10 includes a light-emitting source 1, a reflector 2, a light guide plate 3, a diffusion sheet 4, a first optical sheet 5 and a second optical sheet 6.
The light-emitting source 1 is an element that generates visible ray, and a light-emitting diode (LED) and a cold cathode fluorescent lamp (CCFL) can be selectively used as such a light-emitting source 1.
Light emitted from the light-emitting source 1 is incident on the light guide plate 3 and proceeds inside the light guide plate 3 while generating the total reflection, and since light which is incident on the inner surface of the light guide plate 3 at an incidence angle smaller than an critical angle is transmitted without being totally reflected, the light is emitted to the top and the bottom.
At this time, the reflector 2 improves the light efficiency by reflecting the light emitted to the bottom and causing the light to enter the light guide plate 3 again.
The diffusion sheet 4 diffuses the light emitted through the top surface of the light guide plate 3 to make brightness uniform and broaden the viewing angle, but the light which has passed through the diffusion sheet 4 is degraded in front exit brightness.
The first optical sheet 5 is made up of a base portion 5b and a structural pattern 5a, and emits the light incident from the diffusion sheet 4 by refracting and primarily condensing the light so as to be vertically incident.
Further, the structural pattern 5a is formed integrally with the top surface of the base portion 5b and is made up of a structure for vertically refracting and emitting the light incident through base portion 5b. 
The structural pattern 5a is generally formed to have a triangular cross section, and the vertical angle of the triangular shape is usually substantially 90 degrees.
Moreover, the second optical sheet 6 has the same shape as the first optical sheet 5, and secondarily condenses and releases the light to enhance the brightness of the light that is primarily condensed in the first optical sheet 5.
Here, the first optical sheet 5 and the second optical sheet 6 are disposed and integrally joined together so that an extension direction of the structural pattern of the first optical sheet 5 and an extension direction of the structural pattern of the second optical sheet 6 intersect with each other at a right angle in order to further enhance the brightness.
However, such a configuration have a problem in which, when joining the first optical sheet 5 and the second optical sheet 6, the shape of the structural pattern 5a is deformed while an upper end portion thereof being joined to the bottom surface of the second optical sheet 6, the length of the cross-section trace changes, or a vertical angle or a inclined angle of an inclined surface changes, and thus, the direction of refraction of light transferred from the lower part is not actually uniform.
In this way, there is a problem in which when the direction of refracting and condensing the light in the structural pattern 5a becomes uneven, the brightness is degraded, and the quality of the backlight unit is degraded accordingly.